Within the field of electrochemistry, there is a well-known type of an electrolytic cell known as a chlor-alkali cell. Basically this is a cell wherein chlorine gas and caustic soda, viz., sodium hydroxide, are produced by passing an electric current through a concentrated salt (brine) solution containing sodium chloride and water. A large portion of the chlorine and caustic soda for the chemical and plastics industries is produced in chlor-alkali cells. The cathodes employed in such chlor-alkali cells are subjected to the corrosive environment of the caustic soda.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically impermeable cation exchange membrane, such as the commercially available NAFION manufactured by the E. I. du Pont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos, which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a noble metal coating to yield what is known in the art as a dimensionally stable anode. One of the unwanted by-products present in a chlor-alkali cell is hydrogen which forms at the cell cathode. This hydrogen increases the power requirement for the overall electrochemical process, and eliminating its formation is one of the desired results in chlor-alkali cell operation.
Fairly recently, attention has been directed in chlor-alkali cell technology to various forms of oxygen (air) cathodes. Such cathodes can result in significant savings in the cost of electrical energy employed to operate chlor-alkali cells. Estimates indicate that there is a theoretical savings of about 25 percent of the total electrical energy required to operate chlor-alkali cells provided that the formation of hydrogen at the cathode can be prevented. In other words, about 25 percent of the electrical energy employed in a chlor-alkali cell is used to form hydrogen at the cathode. Hence, the prevention of hydrogen formation by oxygen reduction at the cathode results in significant savings in the cost of electrical power. This is the major benefit of and purpose for oxygen (air) cathodes.
One problem observed with oxygen (air) cathodes was that when the electric power was shut off, e.g., due to a power outage or due to shutdown of a cell for repair or replacement of a component therein, the operating potential, or operating polarization, increased upon restart beyond the previous operating levels. This resulted in poorer performance, premature cathode failure and increased in the cost of operating and maintaining such cells. The above disadvantages were encountered upon startup of the oyxgen cathodes despite the fact that there was no extraneous change in the operating conditions upon the startup of the cell, viz., the same oxygen cathode was used, the same anode was used, the same anolyte was used, the same catholyte was used and the same oxygen-containing gas, e.g., oxygen or oxygen from which the carbon dioxide had been removed, were employed upon startup. Moreover, the same undesirable increase in operating voltage on restart was noted regardless of whether the cathode was operating on CO.sub.2 -free air or oxygen.
U.S. Pat. No. 4,221,644 to Ronald L. LaBarre is directed to air-depolarized chlor-alkali cell operation methods which are stated to maximize the power efficiency available from such oxygen electrodes while minimizing the voltage necessary to operate such oxygen electrodes. The methods set forth include control of the pressure of the air feed side of the oyxgen electrode, control of the total flow of the air feed side, the humidification of the air feed side of the oxygen electrode and the elimination of CO.sub.2 from the air feed to the oxygen electrode to increase the lifetime of such electrodes as applied to chlor-alkali electrolytic cells. U.S. Pat. No. 4,221,644 is not concerned with how to treat an oxygen (air) cathode during a shutdown, whether by accidental or intentional means. At column 10, lines 46-62 of this LaBarre patent, it is stated that the presence of nitrogen in the air creates problems within the oxygen cathode since it acts as a diluent to thereby decrease the concentration of the oxygen present within the oxygen compartment 24 of the LaBarre electrolytic cell 12. LaBarre states that the nitrogen molecules enter the pores of the cathode 18 and must be diffused back out of the pores since they are not used in the reaction. It is stated that this causes a lack of activity within the porous catalytic areas of oxygen cathode 18 such as to reduce the power efficiency possible and increase the voltage necessary for the operation of such a cell. It is additionally stated that this condition may be reduced to a minimum by increasing the total flow so as to provide ample oxygen supply to the oxygen compartment 24, thus reducing to a minimum the voltage necessary to operate the cell while increasing to a maximum the possible power efficiency from such an electrolytic cell 12.
In view of this statement in the LaBarre patent concerning the result of oxygen cathode exposure to progressively greater relative concentrations of nitrogen as being detrimental and raising the operating voltage requirement of such cells, the discovery of the present invention is truly surprising. The present inventor has discovered that by imposing a nitrogen purge on the cell upon shutdown of the electric power, whether due to a power outage or due to an intentional shutdown, and maintaining the nitrogen during the entire period of the shutdown, it is possible to regain the low voltage of operation which was seen in the cell prior to the shutdown.